This has been the case with some incredibly notable monarchs — Henry VIII was actually the "spare" — but in modern times it has thankfully been less necessary, and so the younger sibling nudges further down the line of succession with every niece and nephew born. However the nature of a "spare" can create all sorts of issues; they are a vital cog in the royal machine as they grow up, but as soon as the the elder sibling starts having children of their own, their official role becomes less defined, with (as we have seen in the last five generations of the royal family) very mixed results.